make sure to unlock the screenshot's buffer on destruction

this fixes a memory leak on various devices

Bug: 8475134
Change-Id: I25924674c803a0cb22371a95c60664c36acb0340

include/gui/SurfaceComposerClient.h

@@ -170,6 +170,7 @@ private:
 
 public:
     ScreenshotClient();
+    ~ScreenshotClient();
 
     // frees the previous screenshot and capture a new one
     status_t update(const sp<IBinder>& display);

libs/gui/SurfaceComposerClient.cpp

@@ -627,6 +627,10 @@ ScreenshotClient::ScreenshotClient()
     memset(&mBuffer, 0, sizeof(mBuffer));
 }
 
+ScreenshotClient::~ScreenshotClient() {
+    ScreenshotClient::release();
+}
+
 sp<CpuConsumer> ScreenshotClient::getCpuConsumer() const {
     if (mCpuConsumer == NULL) {
         mCpuConsumer = new CpuConsumer(1);